An established method of analyzing the composition of organic substances, e.g., foodstuffs or polymers, is to record the reflection or transmission of infrared or near-infrared radiation. Infrared radiation or near-infrared radiation excites vibration modes in the material to be examined, which results in characteristic absorption bands. In order to be able to use the complete bandwidth of the near-infrared radiation, light sources having a broad continuous emission spectrum in the desired spectral range are required. The demand for a small, handy and portable analytical apparatus/spectrometer is considerable specifically for analysis carried out by end users, for which reason a small light source is also required. A compact, broadband infrared light source is also required for sensor applications in portable apparatuses and industrial machines. Particularly for applications in the near-infrared range, light sources having a broad continuous emission spectrum are required because this spectral range can be covered by a silicon detector which can be produced in a favorable manner.
Tungsten halogen lamps were hitherto frequently used for the near-infrared range. These lamps are inexpensive and provide a continuous emission with a high intensity in the near-infrared range. However, tungsten halogen lamps have numerous properties which make them disadvantageous for small, handy and portable spectrometers. For example, they generate a great deal of heat, which makes them unattractive for portable spectrometers and the emission wavelength changes over time so that tungsten halogen lamps have only a short service life in comparison with, e.g., LEDs. On the other hand, a certain proportion of emitted radiation is outside the desired wavelength range, as a result of which this energy is lost. Furthermore, they require a considerable volume by reason of the glass casing.
Laser diodes are also used for analyzing materials. In this case, the emission wavelength of the laser diode is selected according to a specific absorption band of the material. Therefore, this method can be used to detect only one component and therefore spectrometers comprising laser diodes can be used only for detecting known substances, such as, e.g., water, carbon dioxide or volatile organic substances. If unknown substances or mixtures are to be analyzed, a broad emission spectrum of the light source is required.
In principle, it is possible to use semiconductor chips which emit radiation in the infrared range. Since previous LEDs which emit radiation in the infrared range have only a very narrow emission width, typically below 50 nm, and since the spectrum typically extends in a peak-like manner, slight changes in the emission wavelength, e.g., as a result of temperature effects, also already have a significant influence upon the intensity of a given wavelength position. In order to cover broader wavelength ranges, a plurality of infrared LEDs would have to be used which must be activated electronically in an individual manner in order to be able to counteract changes in the wavelength of the emission bands and the intensity at elevated temperatures. Moreover, optical elements would be required in order to achieve a mixture of the emission of the radiation of the different LEDs.
Previous light sources frequently have a broadband spectrum with an excessively abrupt or steep change in the intensity or with a strong variation of the spectrum during a sensor measurement or over the service life of the sensor and therefore it is not possible to provide a reliable sensor measurement. Therefore, such light sources are not suitable for precisely analyzing and determining a spectral fingerprint.
Known materials such as Cd (Te,Se) material systems, e.g., quantum dots, satisfy the required criteria of a broad and continuous emission spectrum in the near-infrared range, but they are not suitable for an industrial application owing to the toxicity of cadmium.
Infrared light sources are also used in endoscopes. In conventional endoscopes, 300 W-xenon lamps are used with a filter in order to obtain infrared radiation with a peak wavelength of 785 nm+/−20 nm. A disadvantage of this solution is the considerable losses through the filter and significant heat development and rapid aging of the lamp. Moreover, after being switched on the xenon lamp cannot operate immediately at full radiant power, resulting in waiting times before the endoscope can be used.